refactor: Use regular functions rather than FromStr impls (#8178)

Implementing `FromStr` and then calling `parse()` creates an
indirection, which is hard to follow for people who are not familiar
with Rust. @r10s recently voiced this problem when we were
pair-programming, and I agree.

We can decide what exactly to call the new function.

I didn't remove all `FromStr` implementations yet; `FromStr for
Fingerprint`, `FromStr for Params`, `FromStr for MozConfigTag` and
`FromStr for EphemeralTimer` are still left.

src/mimeparser.rs

@@ -2127,7 +2127,7 @@ async fn parse_gossip_headers(
     let mut gossiped_keys: BTreeMap<String, GossipedKey> = Default::default();
 
     for value in &gossip_headers {
-        let header = match value.parse::<Aheader>() {
+        let header = match Aheader::from_str(value) {
             Ok(header) => header,
             Err(err) => {
                 warn!(context, "Failed parsing Autocrypt-Gossip header: {}", err);